diff --git a/pkgs/maps/main.fnl b/pkgs/maps/main.fnl
index 18a98b6..e0077f0 100644
--- a/pkgs/maps/main.fnl
+++ b/pkgs/maps/main.fnl
@@ -211,6 +211,24 @@ label.readout {
         (when (not (. tbl k))
           (tiles.fetch cq x y zoom #(tset tbl k $1)))))))
 
+;; iso 9899:1999 says
+;; "The atan2 functions return arctan y/x in the interval [−π , +π ] radians"
+
+(fn clamp-angle [rad]
+  (let [circle (* 2 math.pi)]
+    (if
+     (< rad (- math.pi)) (clamp-angle (+ circle rad))
+     (> rad  math.pi) (clamp-angle (- rad circle))
+     rad)))
+
+(define-tests :clamp-angle
+  (expect= (clamp-angle 2) 2)
+  (expect= (clamp-angle (+ (* 4 math.pi) 2)) 2)
+  (expect= (clamp-angle (- 2 (* 4 math.pi))) 2))
+
+(fn deg->rad [deg]
+  (* math.pi (/ deg 180)))
+
 (fn draw-onto-map-surface [surface bounds zoom]
   (let [{ : num-tiles-x : num-tiles-y } bounds
         road-width 14